Generally, rotary compressors are set to different predetermined compression capacities depending on models, and in order to reduce their cost as low as possible, capacities thereof are adjusted by changing an eccentricity amount of a drive shaft and an outer diameter of a roller without changing a shape of a cylinder of the compressor. In this case, however, while the cylinder can be made common between different models, the control of parts becomes complicated because the kinds of drive shafts and rollers increase in number. In addition, there has been such a problem that changes in their production line and changes in centering become necessary, resulting in cost increase.
Furthermore, while there is known an inverter controlled type rotary compressor which is made variable in its compression capacity by control of the number of revolutions to achieve the use of the same of parts, because such an inverter-controlled compressor is very expensive, a refrigeration apparatus incorporating such a compressor becomes very expensive in manufacturing cost.
As a result, as another means for adjusting the compression capacity of the rotary compressor, there is conventionally known, as described in Japanese Utility Model Application Laid-Open Publication No. 54-29403, a method constituted so that a thin plate is inserted between a cylinder and a front head or a rear head, a bypass passage communicating, at starting time of suction, a suction chamber in which a suction port of the cylinder opens with a compression chamber in which a discharge port opens is formed on said thin plate and the compression capacity is adjusted by shifting a suction shut-off position of suction gas toward the compression chamber side.
Namely, said capacity adjustment method is constructed as follows. As shown in FIG. 6, in a rotary compressor wherein, in a cylinder chamber A1 of a cylinder A arranged between a front head and a rear head, a roller B is internally installed with an eccentric portion C1 of a drive shaft C being fitted on said roller B, and on an intermediate position between a discharge port A2 and a suction port A3 formed on said cylinder A. A blade D divides an inner space of said cylinder chamber A1 into a compression chamber X communicating with said discharge port A2 and a suction chamber Y communicating with said suction port A3 and the blade D reciprocably mounted. A rear end of said blade D is urged by a spring D1 so as to contact a tip end thereof with an outer circumference of said roller B at all times. A circular thin plate E having the same diameter as that of said cylinder A and having a shaft hole E2 pierced by said drive shaft C at the center thereof is inserted between said front head and said cylinder A. A bypass passage E1 shifting the suction shut-off position of suction gas sucked from said suction port A3 into said suction chamber Y toward said compression chamber X side is formed on this thin plate E. Said bypass passage E1 is formed in a long circular arc shape along an inner wall of said cylinder chamber A1 and said bypass passage E1 is formed through the thickness of said thin plate E.
Accordingly, with said configuration, the internal space of said cylinder chamber A1 is partitioned, by the blade D and a contact point O in which the outer circumference of said roller B is brought into contact with the inner wall face of said cylinder chamber A1, into a suction chamber Y formed between the contact point O and a wall face of the blade D in the forward side of the rotation direction of said eccentric shaft portion C1 and a compression chamber X formed between said contact point O and a wall face of the blade D in the backward side of the rotation direction of said eccentric shaft portion C1. When driving the drive shaft C, said contact point O of the roller B with the inner wall of said cylinder chamber A1 is moved along the inner wall of said cylinder chamber A1 and gas is sucked into said suction chamber Y from said suction port A3 and compressed in said compression chamber X to be discharged from said discharge port A2, and thus, the suction and compression of gas is repeated. In addition, because said thin plate E is inserted between said cylinder A and front head, when said contact point O is positioned at the bypass passage E1 formed on said thin plate E, said compression chamber X and said suction chamber Y are communicated with each other and the gas within said compression chamber X is not compressed. First when said contact point 0 has passed the bypass passage El, suction of suction gas is shut off and the suction chamber Y and compression chamber X are partitioned in a hermetically sealed state, and thus the compression of gas in the compression chamber X is started. Therefore, by shifting the shut-off position of suction gas to the compression chamber by an arbitrary amount toward the compression chamber X side through changing the length of said bypass passage El, the start time-point of gas compression in the compression chamber X is adjusted and thus, the compression volume in the compression chamber X can be adjusted. In other words, the compression capacity in said compression chamber X becomes controllable arbitrarily and the variation in the capacity of said rotary compressor is enlarged.
However, in said configuration, because a thin plate provided with said bypass passage E1 is required separately, the number of parts is increased and assembly man-hours thereof are increased that much, resulting in complication of the over-all construction. In addition, because said bypass passage E1 has only the passage area corresponding to the thickness of said thin plate E inserted between said cylinder A and front head, not only it is necessary to guide suction gas sucked into the suction chamber Y from said suction port A3 to an axial direction end side of said cylinder A where said thin plate E is arranged but also the resistance when the suction gas within said suction chamber Y passes through said bypass passage E1 is increased, resulting in the problem that an exact control of the compression capacity becomes difficult.